1. Field of the Invention
Polymers having high-temperature characteristics are required to improve the performance and to reduce the weight of industrial materials in electronic devices, aeronautical equipment and some machinery. The polyimides and polyacrylates are polymers known to have the required mechanical strength, dimensional stability, low coefficient of thermal expansion, and electrical insulation properties in addition to high-temperature resistance.
The preparation of high performance polymers, however, requires cure temperatures in excess of 200xc2x0 C. This leads to high tooling costs, high processing costs, and processing induced thermal stresses that can compromise component durability. The process of this invention allows the curing of high performance polyimides and polyacrylates at or near room temperature. This invention enables the cure of high performance polyimides and polyacrylates at or near room temperature by using ultraviolet light or some other radiation sources, such as electron beams rather than heat to provide the cure energy. Specifically, this invention relates to the Diels-Alder cyclopolymerization of a photochemically generated diene with a dienophile, such as bismaleimide and mixtures thereof with a maleimide end-cap and trismaleimide. Irradiation of an aromatic diketone produces two distinct hydroxy o-quinodimethane (photoenol) intermediates. The intermediates are trapped via a Diels-Alder cycloaddition with appropriate dienophiles, e.g., bismaleimide and/or trismaleimide to give the corresponding polyimides in quantitative yields. When maleimides such as bismaleimide and/or trismaleimide are used as the bisdienophile, the resulting polyimides of this invention have glass transition temperatures, (Tg), as high as 300xc2x0 C.
2. Description of the Prior Art
High performance polymers such as polyimides or polyesters are typically prepared by condensation reactions. In the case of polyimides, the reaction involves diamines and dianhydrides or dianhydride derivatives e.g., the diester of tetracarboxylic acids. This process suffers from several problems in that aromatic diamines are toxic, mutagenic, or carcinogenic. Safe handling and disposal of these materials requires the implementation of costly engineering controls. Further, processing of condensation reaction systems also can be a problem since this chemistry leads to low molecular weight by-products, e.g., water and alcohols. Evolution of these by-products and high processing temperatures lead to voids and defects in the polymer and composites prepared with these polymers.
It is known that some of these processing problems can be overcome, however, by combining addition chemistry with condensation chemistry, as is the case for PMR-15 polyimides. With this approach, low molecular weight oligomers (short chain polymers) are prepared by the condensation of diamines with dianhydrides or its derivatives and a suitable endcapping group. The endcaps undergo a cross-linking reaction at high temperatures (typically in excess of 300xc2x0 C.) to provide a polymer network having good solvent resistance and high temperature performance. Prior to cross-linking, however, the imide oligomers are somewhat fluid, and volatile condensation by-products can be removed from the polymer. While this approach overcomes some of the processing difficulties, it requires higher processing temperatures and monomer toxicity is still a concern.
It is known also in the prior art that the Diels-Alder polymerization reaction has been used to prepare high performance polymers such as the polyimides and polyesters. Typical Diels-Alder reactions used to obtain polyimides have involved the reaction of bismaleimides with a suitable bisdiene such as a bisfuran. Other Diels-Alder reactions use a bisdiene precursor, such as bis(benzocyclobutane), to form the bisdiene upon heating to temperatures of 250xc2x0 C. or higher. Using these Diels-Alder cyclopolymerization reactions overcome the health and safety problems associated with other methods of preparing polyimides, since these reactions do not involve the use of aromatic amines as one of the reactants. However, these methods still require high cure and processing temperatures; see, for example, U.S. Pat. Nos. 5,338,827; 5,322,924; 4,739,030 and the Annual Reviews in Materials Science, 1998, 28, 599-630 by M. A. Meador.
The unique feature of this invention is that it employs energy from ultraviolet light, rather than heat to form the polymers. While other radiation curable polymers have been developed, these methods employ either free radical or cationic-based polymerization chemistries. The present invention utilizes photochemically generated dienes (not free radicals or carbocations) and standard Diels-Aider cycloaddition chemistry in the polymerization process.
More specifically, this invention relates to polyimides and to the method of preparing polyimides derived from the photochemical cyclopolymerization of stoichiometric amounts of at least one aromatic diketone selected from the group consisting of: 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g., p-methoxy phenyl, p-tolyl, p-cyano-phenyl, and R is the same or a different radical selected from the group consisting of aromatic radicals, substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and R2 radicals where R1 and R2 are the same or different organic radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons e.g. 1 to 4 carbons, aryl and substituted aryl radicals, and x in the diketone formula is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, xe2x80x94CH2, alkyl radicals of 1 to 8 carbons, ether radicals, ester radicals, aryl radicals and substituted aryl radicals with at least one dienophile selected from the group consisting of bismaleimide, trismaleimide and mixtures of a maleimide end-cap with bismaleimide and/or trismaleimide in various molar ratios to obtain polyimides having glass transition temperatures (Tg) as high as 300xc2x0 C., high thermal-oxidative stability and decomposition-stability temperatures ranging up to about 350xc2x0 C.
Accordingly, it is an object of this invention to employ energy from ultraviolet light rather than heat to obtain polyimides having glass transition temperatures as high as 300xc2x0 C.
It is another object of this invention to provide a novel method of preparing polyimides at ambient temperatures by using radiant energy to photochemically cyclopolymerize aromatic diketones and one or more dienophile.
It is another object of this invention to provide a method of preparing radiation curable polyimides that do not have the health risk associated with conventional methods that utilize toxic aromatic diamines.
It is a further object of this invention to provide polyimides, and a novel process of preparing cured polyimides by using radiation energy at ambient temperatures to polymerize at least one aromatic diketone and a dienophile without using free radical or cationic polymerization methods.
These and other objects of this invention will become apparent from a further and more detailed description of the invention as follows:
This invention enables the curing of high performance polymers at or near room temperature by using ultraviolet light (or some other radiation sources, such as electron beams) rather than heat to provide the cure energy. In general, the invention involves the Diels-Alder cyclopolymerization of photochemically generated bisdienes with dienophiles, such as bismaleimides. The general chemistry is described in Scheme 1, for a representative polyimide. The irradiation of an aromatic diketone produces two distinct hydroxy o-quinodimethane (photoenol) intermediates. These intermediates are trapped via a Diels-Alder cycloaddition with appropriate dienophiles, e.g., bismaleimide, added prior to irradiation, to give the corresponding polymers in quantitative yields. For example, when bismaleimides are used as the bisdienophile, the resulting polyimides have glass transition temperatures, (Tg) as high as 300xc2x0 C. depending upon the structures of the diketone and the bismaleimide. More important, recent work has demonstrated that polyimide films can be prepared by ultra-violet radiation of high solids content varnishes of the appropriate monomers in a small amount of solvent, e.g. cyclohexanone, dimethyl formamide, N-methylpyrollidone and the like.
The general chemistry for the preparation of either polyesters or polyimides from Diels-Alder trapping of photochemically generated bisdiene intermediates is shown (Scheme1) as follows: 
For purpose of this invention, the other diketones that can be used in preparing the polyimides (as in Scheme 1) include the following seven diketones: 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g. p-methoxy phenyl, p-tolyl p-cyano-phenyl, and R is the same or a different radical selected from the group consisting of aromatic radicals, substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons, e.g. 1 to 4 carbons, aryl and substituted aryl radicals, and X in the diketone formulae is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, xe2x80x94CH2, primary, secondary or tertiary alkyl radicals of 1 to 8 carbons, aryl or aromatic radicals, substituted aromatic radicals, primary, secondary or tertiary ethers, poly(ethers), ester radicals, and poly(aryls), having the formula: 
wherein n has the value of 1 or 2, and X in the poly(aryl) formulae is a lower alkyl substitutent or nil.
In-addition to bismaleimides, the trismaleimides can be used as the dienophiles either alone or as mixtures with a maleimide end-cap and/or with bismaleimides as a mixture in stoichiometric molar ratios. Structures of these trisdienophiles include, for example: 
wherein X in the trismaleimide formulae is selected from the group consisting of nil, oxygen, CH2, and xe2x80x94Cxe2x95x90O.
More specifically, the polyimides of this invention are derived from the photochemical cyclopolymerization at ambient temperatures of approximately stoichiometric amounts of at least one
(a) aromatic diketone selected from the group consisting of: 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g. lower alkyl substitutents and R is the same or a different radical selected from the group consisting of aromatic radicals, e.g. substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons e.g. 1 to 4 carbons and aryl and substituted aryl radicals, and X in the diketone formulae is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, xe2x80x94CH2, alkyl radicals of 1 to 8 carbons, ether or poly(ether) radicals, ester radicals, and aryl or poly(aryl) radicals with at least one
(b) dienophile selected from the group consisting of bismaleimides, trismaleimides and mixtures of maleimide with bismaleimides and/or trismaleimides in effective molecular or equivalent ratios i.e. 0 to about 25 molar percent of the endcap maleimide with bismaleimides and/or trismaleimides to obtain polyimides having glass transition temperatures (Tg) ranging up to about 300xc2x0 C., high thermal-oxidative stability and decomposition-stability temperatures ranging up to about 350xc2x0 C.
More specifically, the polyimides of this invention are derived, preferably, by a process of photochemically cyclopolymerizing with ultra-violet light at ambient temperatures approximately stoichiometric amounts of an
(a) aromatic diketone having the formula: 
wherein Ar is the same or a different aromatic or substituted aromatic radical and R is the same or different radical selected from the group consisting or aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons, and aryl radicals, and
(b) at least one bis(maleimide) selected from the group consisting of: 
wherein X in the above bis(maleimide) formula is selected from the group consisting of oxygen, Cxe2x95x90O, SO2, CH2, nil, ether radicals, poly(ether) radicals, ester radicals, polyester radicals, aromatic and poly(aromatic) radicals, and R is selected from the group consisting of alkyl(primary, secondary, or tertiary) radicals, ether radicals, poly(ether) radicals, ester radicals, and poly(ester radicals).
The following examples illustrate the novel process of obtaining either polyimides or polyacrylates by photochemically cyclopolymerizing diketones and dienophiles at ambient temperatures.